Solid-state image sensors, in particular for example CMOS image sensors are known in the art. Many modern CMOS sensors are overlaid with a colour filter array (CFA) for obtaining three (or more) colour components, e.g. red, green and blue. Commercially the most commonly used CFA configuration is the so called “Bayer configuration”. It has a pattern of four pixels arranged in a square, two opposite corners being taken by green pixels, the other two corners being taken by a red pixel and a blue pixel, respectively.
As is well known in the art, during imaging, focusing the lens in a correct position with respect to the image sensor is important for capturing sharp images. An important aspect of modern digital cameras is auto-focus, where the focussing of the lens is performed automatically.
Several companies have sensors and/or cameras on the market provided with “drop-in” pixels that can function as “auto-focus” pixels. That means that some of the standard (image) pixels are replaced by auto-focus pixels, which are dedicated for focusing. Examples of such sensors can be found in commercially available cameras from e.g. Fuji, Nikon, Canon.
U.S. Pat. No. 8,009,223(B2) describes a digital camera having an image sensor with both “image pixels” and “focus-detection pixels” (also referred to as “focus-pixels”). The focus-pixels are different from the neighbouring image pixels, and are designed for providing information for automatically focusing the lens. The focus-pixels do not contribute to the normal shooting of an image, i.e. they do not provide actual image data. An algorithm is described for determining defocus amount, and for adjusting the position of the lens accordingly.
Such focus-pixels have the big advantage that they are located on the sensor itself (no extra optics, no extra hardware required) and that they allow the complete auto-focusing system built around it to work extremely fast, which is particularly important for video capturing.
It should be noted that the auto-focus pixels should preferably work under all kinds of illumination levels, also in very low-light conditions, which is a problem of present image sensors. In currently known imagers, once the light input comes below a pre-determined threshold, the auto-focusing does not work correctly anymore.